1. Field of the Invention
The present invention relates generally to gas turbine engine, and more specifically for a multiple piece air cooled turbine blade or vane.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
The first stage rotor blade and stator vanes are exposed to the highest gas stream temperatures, with the temperature gradually decreasing as the gas stream passes through the turbine stages. The first and second stage airfoils (blades and vanes) must be cooled by passing cooling air through internal cooling passages and discharging the cooling air through film cooling holes to provide a blanket layer of cooling air to protect the hot metal surface from the hot gas stream.
Higher turbine inlet temperatures can be used if the first stage turbine airfoils can be made from certain high temperature resistant metals such as tungsten, molybdenum or columbium. However, these metals have such high melting temperatures that they cannot be cast using the standard investment casting process which have a temperature limit of around 3,000 degrees F. the furnace, the dies and the liquid metal pouring devices used to cast the parts are all limited to this temperature. Also, these metals are very hard and therefore cannot be machined using standard metal machining processes. Thus, a thin wall airfoil cannot be formed from investment casting or standard metal machining processes.